1. Field of the Invention
This invention relates to an image detecting device and, more particularly, an imaging lens system in such a device, having a wavelength selectivity for compensating an optical system containing a photoreceptor in color sensitivity.
2. Description of the Prior Art
In the prior art image detecting devices such as copying machines or the like, including a photosensitive drum of CdS type and a halogen lamp as a light source for illuminating originals, it has been found that red-colored letters and figures on the originals are faintly or not completely copied under the combined influences of the spectral sensitivity in the photoreceptor and the spectral wavelength characteristics in the light source. This is due to the fact that the halogen lamp is usually used at a filament temperature in the range of about 3000.degree. K. with the radiation energy having its peak in the infrared range of 800 to 900 m.mu. and being uniformly decreased toward shorter wavelength range while at the same time the CdS photoreceptor has its spectral sensitivity increased in the range of between infrared and near infrared, so that the red-colored letters and figures will be subjected to excessive exposure in comparison with that of blue- and green-colored letters and figures.
On the contrary, it is difficult to effect good copying of blue-colored letters and figures in a photoreceptor of selenium type.
It is desirable to compensate the color sensitivity in such optical systems containing photoreceptors, light sources, mirrors or the like upon copying. To this end, the prior art has proposed a color filter in the form of a flat plate and a laminated interference film as described in U.S. Pat. No. 4,132,477. Both the proposals are utilized to avoid any excessive exposure by attenuating light in a selected wavelength range, for example, in the near infrared range for the combined system of the CdS type photoreceptor and halogen lamp.
Where the color filter in the form of a flat plate is used, however, there is an aberration in parallel planes and also a loss of light quantity in the effective wavelength range other than the near infrared range due to the characteristics of the filter. Particularly, where a lens of in-mirror type is used with such color filter, a loss of light quantity is increased since the light is passed through the filter two times. Furthermore, the added filter leads to an increased cost.
The laminated interference film also leads to an increased cost since it requires a deposition step. Moreover, the laminated interference film has another disadvantage in that the spectral characteristics thereof is varied depending on various factors, particularly, incident angle in that optical system.
It is further known that a heat absorbing filter for shutting off the near infrared light is used, for example, as a condenser lens for slide projectors. This filter is utilized to prevent the increasing of temperature, but not to provide any wavelength selectivity in a broad wavelength range corresponding to the spectral sensitivity of a photoreceptor as in this invention. Furthermore, the heat absorbing filter will not be used as an imaging lens having a pre-selected optical properties.